Getting to Know the OSI Model for the CCNA Exam

The CCNA exam asks you to provide at least three reasons that the "industry" uses layered interconnection models. Examples of layered networking models include the seven-layer OSI model (which you need to know inside and out) and the Department of Defense (DOD) five-layer model (which you don't). The basic reason for using a layered networking approach is that a layered model takes a task, such as data communications, and breaks it into a series of tasks, activities, or components, each of which is defined and developed independently.

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a guideline and framework, not a rigid standard that manufacturers can usewhen creating their products.The layers of the OSI modelUnder its official name, the Open Systems Interconnection Reference Model, or theOSI model, was developed by the International Organization for Standardization,which uses the abbreviation of ISO. And, yes, the full acronym of the OSI is ISOOSI.The OSI model is a layered model that describes how information moves from anapplication program running on one networked computer to an application programrunning on another networked computer. In essence, the OSI model prescribes thesteps to be used to transfer data over a transmission medium from one networkeddevice to another. The OSI model is a seven-layer model developed around fivespecific design principles:Whenever a discrete level of abstraction is required, a new layer should becreated.•Each layer of the model should carry out a well-defined function.•The function of each layer should define internationally standardized protocols.•The boundaries of the layers should be placed to minimize the flow ofinformation across interfaces.•There should be a sufficient number of layers defined to prevent unnecessarygrouping of functions and the number of layers should also be small enough sothat the model remains manageable.•Moving down through the layersThe OSI model breaks the network communications process into seven separatelayers. From the top, or the layer closest to the user, down, these layers are:Layer 7, Application: The Application layer provides services to the softwarethrough which the user requests network services. Your computer applicationsoftware is not on the Application layer. This layer isn't about applications anddoesn't contain any applications. In other words, programs such as MicrosoftWord or Corel are not at this layer, but browsers, FTP clients, and mail clientsare.•Layer 6, Presentation: This layer is concerned with data representation andcode formatting.•Layer 5, Session: The Session layer establishes, maintains, and manages thecommunication session between computers.•Layer 4, Transport: The functions defined in this layer provide for the reliabletransmission of data segments, as well as the disassembly and assembly of thedata before and after transmission.•Page2of5Getting to Know the OSI Model for the CCNA Exam-For Dummies4/29/2009http://www.dummies.com/how-to/content/getting-to-know-the-osi-model-for-the-ccna-exa...Layer 3, Network: This is the layer on which routing takes place, and, as aresult, is perhaps the most important OSI layer to study for the CCNA test. TheNetwork layer defines the processes used to route data across the network andthe structure and use of logical addressing.•Layer 2, Data Link: As its name suggests, this layer is concerned with thelinkages and mechanisms used to move data about the network, including thetopology, such as Ethernet or Token Ring, and deals with the ways in whichdata is reliably transmitted.•Layer 1, Physical: The Physical layer's name says it all. This layer defines theelectrical and physical specifications for the networking media that carry thedata bits across a network.•Other interesting OSI layer stuffLayers 5 through 7 are generally referred to as the upper layers. Conversely, Layers1 through 4 are collectively called the lower layers. Seems obvious, but you'll seethese references on the test.You need to know the seven layers in sequence, either top-to-bottom or bottom-to-top. Here are some mnemonic phrases to help you remember the layers of the OSImodel:"Please Do Not Throw Salami Pizza Away" — this works for bottom-to-top. Ifyou don't like salami pizza, then how about seafood or spinach pizza instead?•"All People Seem To Need Data Processing" — a top-to-bottom reminder.•"APS Transports Network Data Physically" — APS refers to Application,Presentation, and Session. This one separates the upper and lower layergroups.•"Please Do Not Tell Secret Passwords Anytime" — Shh! Another bottom-to-top phrase.•Packaging the